Objective: To describe and compare the perceptions of the general population about the harmful effects of electronic cigarettes (e-cigarettes) on users and on those passively exposed to e-cigarettes and the perceptions about e-cigarette usefulness for reducing or eliminating tobacco smoking. Design, setting, and participants: We analysed cross-sectional data from a longitudinal study of a representative sample of the general adult (≥16 years) population of Barcelona, Spain (336 men and 400 women). The fieldwork was conducted between May 2013 and February 2014. We computed the percentages, adjusted OR and their corresponding 95% CI among participants with some awareness of e-cigarettes (79.2% of the sample). Primary and secondary outcome measures: We assessed the perception about harmfulness for e-cigarette users and for passively exposed non-e-cigarette users, as well as the perception of usefulness for smokers of cigarette cessation and reduction.